Every excimer laser requires at least two mirrors, namely a rear mirror and a front mirror. When these mirrors are directly connected to a laser vessel to seal the same, the rear mirror typically consists of an element of suitable transparent material (such as quartz, magnesium fluoride or calcium fluoride), the outer surface of which, i.e. the surface more remote from the lasing cavity, is coated to be substantially fully reflective. The front mirror is a similar element of similar material, but whose outer surface is coated to be only partly reflective to ensure sufficient round trips of the beam in the lasing cavity, while being mainly transmissive to provide the output beam. An alternative technique uses two transparent windows to seal the vessel, while mounting front and rear mirrors external to the windows. In each case accurate alignment of the mirrors is essential for efficient operation of the laser and for ensuring an accurate pointing direction of the output beam towards a target.
For the purposes of this specification including the claims, the term "mirror" is used to denote the elements that provide the optical feedback to the laser cavity, i.e. they reflect at least a portion of the laser beam back along the laser axis, whether or not they also serve the function of sealing the laser vessel; the term "window" is used to denote the essentially transparent elements that may instead be used for sealing the laser vessel; and the term "optic" is used generically to refer both to such mirrors and windows.
In many other (i.e. not excimer) lasers, the mirrors are mounted, adjusted and firmly fixed in place by the manufacturer, and they continue to serve both optical and sealing functions for the life of the laser without requiring any further adjustment or attention by the user.
Excimer lasers, however, present special problems in this respect.
First, by virtue of the fact that an excimer laser is operated as a closed system (no gas flowing in or out during operation) and at a superatmospheric pressure, e.g. about 4 atmospheres, the inevitable temperature and pressure variations in the laser vessel set up such distortions and strains in the vessel that the ends of the vessel are caused to vary in position relative to each other and hence upset the alignment of mirrors mounted at these ends. One proposal for overcoming this problem employs the alternative technique mentioned above and mounts the mirrors on a rigid frame external to and mechanically independent of the end portions of the laser vessel, the function of sealing the vessel ends being taken over by the transparent windows. This use of four optics, two mirrors and two windows, has the disadvantage that it reduces the output power of the laser by typically about 20%.
The second problem presented by excimer lasers resides in the fact that the impurities generated in the laser vessel during operation necessitate frequent cleaning, typically at least once a week, of the surfaces of the mirrors or windows that are in contact with the gases in the laser vessel. Hence it is impossible for the manufacturer to adjust and fix all these optics permanently in place. The user has to be able to remove the optics (mirrors or windows) for cleaning and remounting or replacement with a new optic, while retaining the correct original orientations. It is thus important to provide mounts for the optics that are of such a nature that the user, who lacks the equipment or expertise for accurate realignment of the optics, can readily remove and replace the optics without a loss of alignment.
The alignment that has been under consideration so far concerns the adjustment of the axis of the laser beam in the Y and Z directions, where the X direction is the axis of the laser cavity and all these three directions are mutually perpendicular. In almost all excimer lasers each optic or at least one of them is formed with a wedge, i.e. is in the shape of a wedge with an angle between its inner and outer surfaces. While this angle is small, typically only about 20 milliradians or less (and is not normally discernable by eye), rotation of an optic by 180.degree. about the X axis, i.e. complete reversal of the wedge slope, represents a serious optical misalignment. Even rotation of an optic around the X axis by much less than 180.degree. is undesirable, and hence an optic mount is needed that is foolproof in the sense of only being capable of reassembly in the same orientation in respect of rotation about the X axis.